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Peripheral Nervous System Anatomy
Peripheral Nervous System Anatomy
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The Peripheral Nervous System (PNS) includes all neural tissue outside the brain and spinal cord, which together make up the Central Nervous System (CNS). If the CNS is the main control center, the PNS acts as the communication network connecting it to the rest of the body.
The PNS is composed primarily of nerves and ganglia:
Cranial nerves (12 pairs): Originate from the brain; most serve the head and neck, though the vagus nerve extends to thoracic and abdominal organs.
Spinal nerves (31 pairs): Originate from the spinal cord and branch throughout the body, providing sensory and motor innervation.
Each nerve contains bundles of neurons, which transmit electrical impulses.
Afferent (sensory) neurons carry information toward the CNS.
Efferent (motor) neurons carry commands away from the CNS to muscles and glands.
The Three Main Functions of the PNS
Sensing: Collects information from your surroundings (like touch, temperature, and pain) and sends it to your brain.
Movement: Carries brain commands to your muscles so you can move.
Automatic processes: Controls body functions you don’t have to think about, like heartbeat and digestion.
Cranial Nerves
Cranial Nerves
There are 12 pairs of cranial nerves in total. 12 of these are part of the peripheral nervous system (PNS) and connect the brain to different parts of the head, neck, and body.
Each cranial nerve has a specific function: some carry sensory information (like sight or smell), some control muscles (motor function), and some do both (mixed nerves).
Cranial Nerves I and II (Olfactory and Optic) come directly from the cerebrum (forebrain).
Cranial Nerves III–XII come from different parts of the brainstem.
The 12 Cranial Nerves and Their Main Functions
The 12 Cranial Nerves and Their Main Functions
Olfactory Nerve (Cranial Nerve I) – This is a sensory nerve responsible for your sense of smell.
Optic Nerve (Cranial Nerve II) – This sensory nerve allows you to see by carrying visual information from your eyes to your brain.
Oculomotor Nerve (Cranial Nerve III) – This is a motor nerve that controls most eye movements and also helps adjust the size of your pupils.
Trochlear Nerve (Cranial Nerve IV) – This motor nerve helps your eyes move downward and inward.
Trigeminal Nerve (Cranial Nerve V) – This is a mixed nerve, meaning it has both sensory and motor functions. It allows you to feel sensations on your face and helps you chew.
Abducens Nerve (Cranial Nerve VI) – This motor nerve controls the movement of your eyes from side to side.
Facial Nerve (Cranial Nerve VII) – This mixed nerve controls facial expressions and also allows you to taste on the front part of your tongue. It helps produce tears and saliva as well.
Vestibulocochlear Nerve (Cranial Nerve VIII) – This sensory nerve is responsible for hearing and balance.
Glossopharyngeal Nerve (Cranial Nerve IX) – This mixed nerve helps with taste on the back of the tongue and controls muscles used for swallowing.
Vagus Nerve (Cranial Nerve X) – This mixed nerve controls many internal organs, including the heart, lungs, and digestive system. It helps regulate breathing, heart rate, and digestion.
Accessory (Spinal Accessory) Nerve (Cranial Nerve XI) – This motor nerve controls the muscles in the neck and shoulders, allowing you to move your head and shoulders.
Hypoglossal Nerve (Cranial Nerve XII) – This motor nerve controls the muscles of the tongue, allowing you to speak and move your tongue properly.
Spinal nerves
The spinal nerves are the second group of nerves in the peripheral nervous system (PNS).
There are 31 pairs of spinal nerves in total, and each pair connects to a specific section of the spinal cord:
8 cervical nerves (C1–C8) – in the neck region
12 thoracic nerves (T1–T12) – in the upper and mid-back
5 lumbar nerves (L1–L5) – in the lower back
5 sacral nerves (S1–S5) – in the pelvic area
1 coccygeal nerve (Co1) – near the tailbone
Each spinal nerve carries both sensory and motor information, linking the brain and spinal cord to the rest of the body.
How Nerves Work
Neurons come in many sizes and shapes (depending on their job and where
they’re found). Each neuron has:
A cell body (the main part)
Dendrites (short branches that receive messages)
Axons (long branches that send messages)
There’s also myelin, a fatty coating that helps signals move faster.
There are three types of nerve signals:
Sensory (afferent) – carry messages to the brain.
Motor (efferent) – carry commands from the brain.
Autonomic – manage automatic organ functions.
Autonomic Nervous System (ANS)
There are Two Main Systems of the PNS (the ANS and SNS). The ANS regulates involuntary functions of smooth muscle, cardiac muscle, and glands, maintaining homeostasis. It has three components:
Sympathetic System:
Originates in thoracolumbar spinal cord (T1 – L2).
Increases heart rate, dilates pupils, redirects blood flow to muscles, and mobilizes energy.Parasympathetic System:
Originates from craniosacral regions (cranial nerves III, VII, IX, X and spinal segments (S2 – S4).
Promotes digestion, decreases heart rate, and conserves energy.
There’s also a small Enteric System. A semi-independent network within the gastrointestinal tract, controlling peristalsis and secretion.
Often called the “second brain” because it can function autonomously, though it’s influenced by both sympathetic and parasympathetic inputs.
Somatic Nervous System (SNS)
Controls voluntary actions and conscious perception.
Sensory division: Delivers information from skin, muscles, and joints to the CNS (e.g., touch, temperature, pain, proprioception).
Motor division: Sends impulses from the CNS to skeletal muscles, enabling movement and reflexes such as the withdrawal reflex.
Both cranial and spinal nerves contribute to somatic control. Many spinal nerves form plexuses (networks) - including the cervical, brachial, lumbar, and sacral plexuses - that give rise to major peripheral nerves like the sciatic or median nerve.
Ganglia
Ganglia
A ganglion (plural: ganglia) is a small cluster of nerve cell bodies found in the peripheral nervous system (PNS).
You can think of ganglia as relay stations where nerve signals are passed from one neuron to another.
When a signal enters a ganglion, it activates the neurons inside, and the message then continues along to its next destination.
There are two main types of ganglia:
Sensory ganglia – part of the somatic nervous system (SNS)
These contain the cell bodies of sensory neurons that carry information to the central nervous system (CNS).
Examples: Dorsal root ganglia (along the spinal nerves) and ganglia of some cranial nerves.
Autonomic ganglia – part of the autonomic nervous system (ANS)
These are involved in involuntary control of organs and glands.
They include: sympathetic ganglia – located close to the spinal cord; parasympathetic ganglia – located near or inside target organs (like the heart, lungs, or digestive organs).
Need a recap? Take a look at the Peripheral Nervous System anatomy notes here.
Need a recap? Take a look at the Peripheral Nervous System anatomy notes here.
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References
Cleveland Clinic. (2022, May 25). Peripheral Nervous System (PNS): What It Is & Function. Cleveland Clinic; Cleveland Clinic. https://my.clevelandclinic.org/health/body/23123-peripheral-nervous-system-pns
Peripheral Nervous System - TeachMePhysiology. (2023, September 18). TeachMePhysiology. https://teachmephysiology.com/nervous-system/components/peripheral-nervous-system
Rad, A. (2021, October 23). Peripheral nervous system. Kenhub. https://www.kenhub.com/en/library/anatomy/the-peripheral-nervous-system
Smith, M. (2023, October 30). Nerve ganglia. Kenhub. https://www.kenhub.com/en/library/anatomy/nerve-ganglia
TeachMeAnatomy. (2025, April 2). Cranial nerves – Summary. TeachMeAnatomy. https://teachmeanatomy.info/head/cranial-nerves/summary
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